Engine primer source

[RV7 To Go]

I originally purchased the primer solenoid kit while building my 7. During the build I purchased a 4 with no primer. Pumping the throttle 2 or 3 times when cold, using the accelerator pump in the carb for priming, worked fine for 4 years and 450 hrs, winter and summer. I removed the primer on the 7, which I had not flown yet, liking the idea of fewer things to go wrong. After 375 hrs in almost 3 yrs on my 7 and in weather down to -10C (using preheat) starting has never been an issue. FWIW.

[avrojockey]

Originally Posted by RV7 To Go

I originally purchased the primer solenoid kit while building my 7. During the build I purchased a 4 with no primer. Pumping the throttle 2 or 3 times when cold, using the accelerator pump in the carb for priming, worked fine for 4 years and 450 hrs, winter and summer. I removed the primer on the 7, which I had not flown yet, liking the idea of fewer things to go wrong. After 375 hrs in almost 3 yrs on my 7 and in weather down to -10C (using preheat) starting has never been an issue. FWIW.

This ^^^^

Save the weight, and risk inducing complexity and don't install it. I bought a -9A that didn't have a primer and was concerned about it for WI weather. Not an issue at all especially with electronic ignitions ability to ignite very lean mixtures.

Here's the logic to this decision...

1. Carbed vehicles started off accelerator pumps for decades without priming
2. You should preheat your engine below 32F anyway so the transmitted warmth from engine to carb bowl is actually better because it "warms" the fuel providing greater vaporization. Primer fuel won't be warm.
3. For fire you need a fuel...that fuel has to vaporize before igniting (even wood or paper doesn't burn, it needs to heat to vaporize first and mix with O2 to produce a combustible flame). Priming is simply dumping enough fuel in the induction system that enough vapor is created to support combustion. The colder the fuel and air the more primer fuel qty to create the vapor to go bang! Therefore the goal to successful starting should be turning fuel into vapor and regulating air to support combustion, not necessarily more fuel.

I did my insurance checkout with rocketbob and he recommended "pumping" the gas a couple times at the start of cranking. This has worked flawlessly this winter...even without my Pmag installed. My start technique cold...crank and pump throttle 2x, then throttle back all the way to idle to choke off air. Fire right up and idles at 600-700 rpm, then I slowly bring to a smooth idle around 900. Hot starts require NO pump/priming. Crack throttle 1/4" and just crank...starts like a car.

I believe primers are only necessary in our lycs when it comes to hand propping to help "precharge" cylinders with vapor. Much like a hybrid engine does for quick restarts.

[gmcjetpilot]

It is up to the builder. I do recommend as suggested above, if you have no primer, do not pump the throttle until you are cranking the engine.

Some O-320's have carbs ( MA-3A is one ) with out an accelerator pump carburetor. You need a primer system. Just FYI.

Solenoid and some copper tube to one, two or three cylinders is not a huge expense or weight. Put it in if you want. My RV-4 had a primer solenoid. I'd turn boost on, push the button for a second and light the engine. No throttle pumping needed.